Investigation of the biological role of RNA ligase(s) in various mutant strains of phage T4 as well as virus infected and uninfected eucaryotic cells is proposed. A new assay for the enzyme permits its detection even in RNAse containing lysates at low concentration. We propose to use this to determine whether this (or a related) enzyme is responsible for processing RNA in other organisms and to examine putative substrates. The absence of ligase activity in a T4 gene 63 amber mutant raises intriguing problems about its in vivo function which will be pursued. Detailed studies of alternative forms of the enzyme are also proposed, to determine the nature of the differences seen in the enzyme from E. coli MRE600 for example.